linux-kernel - Re: [PATCH 3/4] locking/rwbase: Fix rwbase_write_lock() vs __rwbase_read

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Date:   Tue, 14 Sep 2021 09:45:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mike Galbraith <efault@....de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 3/4] locking/rwbase: Fix rwbase_write_lock() vs
 __rwbase_read_lock()

On Thu, Sep 09 2021 at 12:59, Peter Zijlstra wrote:

> Boqun noticed that the write-trylock sequence of load+set is broken in
> rwbase_write_lock()'s wait-loop since they're not both under the same
> wait_lock instance.

Confused.

lock(); A

for (; atomic_read(readers);) {
   ...
   unlock();
   ..
   lock(); B
}

atomic_set();
unlock(); A or B

The read/set is always in the same lock instance.

> Restructure the code to make this more obvious and correct.

I agree that it's easier to read, but I disagree that it makes the code
more correct.

Thanks,

        tglx